Thermal stresses can be created in self-enclosed heat exchangers (i.e. stacked plate heat exchangers with integral manifolds, where the fluids are self-contained and do not require an outer housing) where manifolds for hot fluids are provided on the outer periphery of a plate stack, while central portions of the plate stack are cooled by circulation of a coolant. The hot fluid manifolds are in contact with the hot fluid and are significantly hotter than the central areas of the stack, which are in constant contact with a coolant. Consequently, there is a significant surface temperature difference at the hot gas inlet manifold between its side adjacent to the peripheral edge of the heat exchanger (outer side) and its side adjacent to the central (main) coolant passage (inner side). Such a thermal gradient in the manifold can result in high thermal stresses at the manifold. A similar issue can occur at the hot gas outlet manifold, however, it can be to a lesser extent, as the gas temperature has typically been reduced upon contact with the heat exchange coolant.
The situation described above can also create a thermal gradient across the plates which may cause thermal stresses. This issue can arise in any situation where a high temperature fluid enters a heat exchanger through uncooled manifolds provided at the outer edges of a plate stack, such as in an EGHR (exhaust gas heat recovery) cooling and charge air cooling, where a hot gas is cooled by a liquid or gaseous coolant.
FIG. 1 shows an example of an EGHR heat exchanger from a related U.S. patent application Ser. No. 13/599,339, filed Aug. 30, 2012, and incorporated herein by reference. In use, the heat exchanger is mounted to an exhaust valve as shown in FIG. 2. The flow of hot exhaust gas and coolant are shown in FIG. 2. An embodiment of the plate of the heat exchanger is shown in FIG. 3. As would be recognized by a person of ordinary skill in the art based on a reading of the specification that although the heat exchanger described herein is with reference to an EGHR heat exchanger, the invention disclosed herein is not particularly limited for use in an EGHR heat exchanger but can be used in separate applications for heat exchange.
Due to design constraints dictated by the valve configuration in an EGHR, and in order to maximize cooling efficiency, the exhaust inlet and outlet manifolds are located at the edges of the heat exchanger core. It will be appreciated that the portions of the stack which are in contact with the coolant will be at a considerably lower temperature than those areas of the stack which are in contact with the hot exhaust gases only (circled in FIG. 2), thereby creating a thermal gradient across the plates making up the stack. In addition, the hot exhaust gas manifold portion located close to the peripheral edges of the heat exchanger plate can be significantly hotter than the hot exhaust gas manifold portion positioned on the inner side of the plate and in contact with the coolant fluid. This can significantly affect the durability of the heat exchanger that is exposed to hot gases, such as the heat exchanger in an EGHR system.
The thermal gradient described with reference to FIG. 2 can result in thermal stresses when the heat exchanger is heated and cooled under normal operating conditions. Also, because the plate stack has hot fluid manifold sections at the plate ends, the hot outer surfaces of the manifolds are exposed to the environment. Sudden contact of the hot outer surfaces of the heat exchanger with water, as when the vehicle is driven in wet conditions, will cause thermal shocks which may produce additional stresses. In addition, when the hot exhaust gas travels along the length of the inlet exhaust gas inlet manifold, the hot exhaust gas impinges directly on the lowest heat exchange base plate at the end of this hot exhaust gas inlet manifold section. As the flow of the hot exhaust gas impinges generally normal to the inlet manifold end portion at the base plate, it leads to a section of the base plate being at a higher temperature than other portions of the base plate, and leads to a thermal gradient and risk of localized material degradation over time due to hot exhaust gas impingement. Moreover, as the hot gas inlet manifold portion of the base plate is cooled to a lesser extent than the cooled core sections of the heat exchanger plates, the thermal gradient and stress on the base plate can be significantly higher.
There is a need in the art for a heat exchanger having uniformly cooled heat exchanger plates and a base plate that can help to reduce the thermal stresses caused by the thermal gradient which results from a hot exhaust gas flowing through the heat exchanger. In addition, there is a need in the art for a means that can help to reduce and/or protect the base plate from the hot exhaust gas impinging on the base plate of a heat exchanger.
Similar reference numerals may have been used in different figures to denote similar components.